The present invention relates to adhesive compositions. More specifically, the present invention relates to epoxy adhesive compositions.
The present invention relates to an epoxy-based adhesive formulation for electronics packaging. As one example, the present invention relates to an epoxy based adhesive formulation for chip-on-flex applications in electronic packaging. Chip-on-flex applications involve the placement of electronic chips onto flexible films, such as polyimide films, to form high density interconnect packages. Chip-on-flex applications are most often used in packaging of semiconductors.
An epoxy for this type of application should be homogeneous and have a good flow for uniform spin coatability, it should remain tacky after application to the flex and after partial cure or solvent removal so that chips or electronic components placed on it will be held in place, it should be laser ablatable so that adhesive-free pockets can be formed to provide areas to position electronic components which may contain sensitive structures or moveable components such as may be found in MEMs devices, it needs to be hydrophobic to prevent significant moisture absorption by the film in its partial cured state (moisture absorption will degrade the films cured properties) so that a long working life of the coated film is possible to allow time for ablation and chip placement, it must have a sufficient thixotropic character to avoid encroaching into the air pocket (if present) under the attached electronic devices and prevent movement of the electronic devices during final cure, it must provide a void-free attach for the electronic components during adhesive cure so that subsequent vias may be laser drilled down to contact pads on the chip components, it must provide good adhesion to both the flex film and the electronic components and it must have adequate absorption from 200 nm to about 400 nm to allow for laser ablation of the air pockets and via interconnects.
Adhesives in chip-on-flex applications are often laser ablated to provide a series of pores in the adhesive layer. Laser ablation provides protection for adhesive-sensitive or pressure-sensitive portions of semiconductor chips such as airbridge structures, which are liable to be crushed, and transmission-line or resonant structures, the properties of which may be affected by proximity to dielectric materials. This laser ablation allows the formation of integral air pockets in adhesive-free regions under devices attached directly to the adhesive surface. A suitable method of laser ablation is demonstrated in U.S. Pat. No. 6,255,137.
Unfortunately, commercially available epoxies produce large amounts of soot during the laser ablation process, leading to impurities in the electronic packaging. Furthermore, many commercially available epoxies do not presently provide adequate flow and thixotropic qualities. In addition, many commercially available epoxies are not sufficiently fast drying.
It would therefore be desirable to develop an improved fast drying epoxy based adhesive formulation which ablates with reduced soot generation, provides minimal encroachment into the ablated regions, and provides good chip adhesion.
According to one aspect of the invention an adhesive composition is provided. The adhesive composition includes epoxidized cashew nutshell liquid, a catalyst, and diglycidyl ether of bisphenol A. The invention may further include at least one additive selected from the group including curing agents, bonding enhancers, hardeners, flexibilizers, tackifiers, and mixtures thereof.
According to a further aspect of the invention, an adhesive formulation suitable for curing is provided. The formulation includes epoxidized cashew nutshell liquid, diglycidyl ether of besphenol A, and a curing agent. At least one additional additive selected from the group consisting of a bonding enhancer, a hardener, a flexibilizer, a tackifier, and mixtures thereof may also be included in the formulation.
According to a third aspect of the invention, an adhesive composition including the reaction product of epoxidized cashew nutshell liquid, diglycidyl ether of bisphenol A, a curing agent, and a catalyst is provided.
A method for forming an adhesive composition is also provided. The method includes the step of mixing together epoxidized cashew nutshell liquid, a catalyst, diglycidyl ether of bisphenol A, and optionally, at least one each of a curing agent, a bonding enhancer, a hardener, a flexibilizer, and a tackifier.
According to another embodiment of the invention, an electronic chip is provided. The chip includes a substrate; an adhesive including epoxidized cashew nutshell liquid, a catalyst, and diglycidyl ether of bisphenol A; and at least one semiconductor device.